The invention relates to the field of Ge or SiGe waveguides for integrated optical circuits and far infrared application, and in particular to making low loss crystal quality waveguides and photonic crystal structures where there is no sidewall scattering loss from etching.
The waveguide structures often referred to when fabricating Ge or SiGe/Si waveguides are two structures: channel waveguides and ridge waveguides. These waveguide structures also come with the shape of bend, ring, microdisk or taper. One exemplary way to make them includes the deposition of one higher index material (core) whose the refractive index is higher than surrounding materials, and then etching the material into the channel (deep etch) or ridge (shallow etch) structure followed by a deposition of surrounding materials. For example, a semiconductor laser generally has a ridge waveguide structure.
The etching process usually defines the dimension of the waveguide. One of the challenges from etching is the formation of rough sidewalls which causes scattering loss. For high index contrast waveguides with dimensions close to or less than the propagation wavelength in the core materials, there is a large amount of scattering loss (>10 dB/cm) and this becomes more of a challenge with the decrease of dimensions used in waveguides.
This becomes a very serious problem for Ge based waveguide since Ge's refractive index is among the highest (˜4.0) which require much smaller size than conventional materials such as Si waveguide. The state of the art high index contrast Si waveguide has a loss of ˜10 dB/cm before any smoothing treatment, with a RMS roughness of >5 nm. The roughness inevitably introduces more loss when the light passes through it. For SiGe with high content Ge, it has been shown that the sidewalls produced by standard dry etching processes are very rough (˜10 nm RMS roughness), leading to a loss of ˜20 dB/cm. Another challenge is the complicated layering in an integrated optoelectronic circuit where the electronic and optical circuits run together in different layers. It is very convenient if one can reduce the number of layers or try to do both circuits functions on the same layer.
High index contrast systems are also ideal for photonic crystal structures. Photonic crystal structure is gaining more and more interest recently because of its unique properties. However, the photonic crystal structures demonstrated up to date still suffer from significant losses although there should be no loss in theory. This is again due to the side wall roughness of the photonic structures resulting from dry etching processes. Therefore, the performance of photonic structures could be greatly improved if the sidewall roughness problem could be solved.
Ge/Si materials are also one of the important far infrared optical materials (wavelength of 8˜12 μm and 3˜5 μm) because there is no absorption at all when the wavelength is larger than 2 um and a major materials for infrared lens. Given the fact that integrated optics is moving into infrared application, there is also need to fabricate the low loss waveguide for infrared application.